Scientists in China, attempting to study the effects of brain chemistry on sexual behavior, bred a group of mice whose brains were not receptive to serotonin, the neurotransmitter associated in humans with feelings of well being. But instead of, say, sleeping all day and abandoning personal hygiene, these mice lost all interest in female sexual partners, and instead mounted their male counterparts. From the BBC:
When presented with a choice of partners, they showed no overall preference for either males or females.
When just a male was introduced into the cage, the modified males were far more likely to mount the male and emit a "mating call" normally given off when encountering females than unmodified males were…
However, a preference for females could be "restored" by injecting serotonin into the brain.
Of course, there are major differences between the sexual behaviors of mice and those of humans. They, for instance, rely much more on smell. (Think to an "Oh-God-is-that-Axe-Body-Spray-you're-wearing-I-need-to-leave-sorry-don't-call-me" degree.) And while there's been some evidence of "altered responses" to selective serotonin reuptake inhibitors in the brains of us homos, it would still be foolhardy to apply these findings to humans.
But still: party at my place. You bring your hot straight friends, I'll bring the serotonin inhibitors. And the dip.